He quickly joined the Polish resistance. After 1940 he became the main organiser of the Akcja N, a secret organisation preparing German-language newspapers and other propaganda material pretending to be official German publications, in order to wage psychological warfare against German troops.

He also served as an envoy between the commanders of the Home Army and the Polish Government in Exile and other allied governments. During his first trips to Sweden and Great Britain he informed the Western governments of the fate of Poland under German and Soviet occupation. He was also the first to report of the Warsaw Ghetto Uprising. During one of such missions, in July 1944, he returned to Warsaw only a few days before the Warsaw Uprising broke out.

During the Uprising he took an active part in the fights against the Germans and also organised the Polish radio that maintained the contact with the Allied countries through daily broadcasts in Polish and English. Shortly before the capitulation of the Polish capital, he was ordered by Home Army's commander-in-chief Tadeusz Bór-Komorowski to leave the city and find his way to London. He managed to evade being captured and reached Great Britain, bringing with him large quantities of documents and photos. For his bravery and his travels through the German-occupied Europe he was awarded with the Virtuti Militari, the highest Polish military medal.

After the war Jan Nowak-Jeziorański stayed in the West, initially in London and then in Munich and Washington. Between 1948 and 1976 he was one of the most notable personalities of the Polish division of the BBC radio agency. In 1952 he also became the head of the Polish section of the Munich-based Radio Free Europe. Through his daily radio broadcasts he remained one of the most popular radio personalities, both in communist-held Poland and among the Polish diaspora in the West. After giving up his posts in 1976 he became one of the most prominent members of the Polish American Congress. He was also working as an advisor to the American National Security Agency and the presidents of the USA Ronald Reagan and Jimmy Carter. Through his contacts with many notable politicians in the USA, he was one of the proponents of Poland's membership in NATO (achieved in 1999).

In the 1990s he started his cooperation with the Polish Radio and wrote a series of broadcasts titled Polska z oddali (Poland from a Distance). Since 1990 he was also present on Polish television as writer/presenter of monthly programs. In July 2002 he returned to Warsaw for the final time. He was an active supporter of Poland's entry into the European Union. Most of his books, published abroad as well as those published in Poland after 1989, were best-sellers and gained him even more popularity.

For his writings he was awarded some of the most prestigious Polish literary awards, including the Kisiel Award (1999), Ksawery Pruszyński Memorial Prize of the Polish Pen-club (2001) and the Superwiktor award for television personalities. In 2003 he was also awarded the Człowiek Pojednania prize by the Polish Council of Christians and Jews for his part in the Polish-Jewish dialogue. Finally, he was made the doctor honoris causa of many Polish universities, including the Warsaw University, Jagiellonian University and his alma mater, the University in Poznań.

He died in Warsaw on 20 January 2005. He donated all his archives to the Ossolineum institute.

Courier from Warsaw (Kurier z Warszawy, published in London 1978, Polish underground edition 1981, official edition in 1989, published in English in 1982 by Wayne State University Press) ISBN 0-8143-1725-1

Ideological competition in United States' strategy, Polish American Congress, 1980.